1. Field of the Invention
The present invention relates to imaging systems responsive to near-IR radiation and, in particular, to such imaging systems which utilize solid-state technologies for the generation and associated processing of radiation-generated charge signals.
2. Description of the Prior Art
The near-infrared (IR) portion of the electromagnetic spectrum bridges the visible (approximately 0.4 .mu.m to 0.75 .mu.m) and middle infrared (3 .mu.m to 6 .mu.m) regions. Although much work has been directed to the development of high resolution - low crosstalk imagers in the latter two segments of the spectrum, the performance and development of similar devices at near-IR is hampered by the relatively deep photon penetration incurred at near-IR. The application of charge transfer technologies, including CCD (charge coupled device) which offer the possibility of multiplexing several detectors of an array onto a single readout line, has been hampered by the necessity of assuring full depletion at near-IR to avoid crosstalk. It has been established that about ten percent of incident 0.9 .mu.m photons remain unabsorbed at a depth of 35 .mu.m in silicon. Yet, the depletion region of a present-day commercial CCD fabricated on an extrinsic substrate is typically 2 .mu.m or 3 .mu.m deep.
The known deep depletion property of intrinsic silicon has led to the extensive application of PIN detectors, for instance, in conjunction with laser target designators. The PIN detectors typically feature a collection node of a first conductivity type created on an intrinsic substrate of a second conductivity type and a highly doped backside layer of the second conductivity type (e.g., n-type diode diffusion, p-type high resistivity substrate, p+ ohmic contact). Arrays utilizing such PIN diodes are presently read out by circuitry which is wire bonded directly to the individual detectors of the array. The use of numerous leads severely limits the number of detectors per array for a given size thereof. Although the wires utilized for readout are nominally 1 mil in diameter, the ultrasonic bonding process commonly employed causes the detector contact area to expand to about 3 mils in diameter.